Why `#[cfg(not(test))]` does not work with `compile_error!{...}`?

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1

This is the minimum example:

#[cfg(not(test))]
compile_error!{"some error"}

I expect that running cargo test won't have that compile error. However, this condition compilation does not work. I suppose the condition #[cfg(not(test))] was not supported, however

#[cfg(not(test))]
pub fn add(){}

when running cargo doc --open, the documentation does have this item while #[cfg(test)] makes the item hidden in the documentation, this result witnesses this condition compilation should work. Why does it not work for the compile_error!{"some error"} item?

2

When you are testing a library, the library is still also compiled without cfg(test), for use in its integration test targets (tests/*.rs) and dependent packages. (cfg(test) is only active when producing a test binary that runs #[test] functions, not in any other case. That test binary is not a library.)

I’d hope that’s skipped if you have none of those, but perhaps you do, or perhaps Cargo isn’t that clever.

3

The cargo book also lists the targets that cargo test builds by default (section Target Selection). These include:

  • lib — used to link with binaries, examples, integration tests, and doc tests
  • lib as a unit test
  • ...

In particular, a library is built twice and only the "lib as a unit test" enables the test compilation flag, as @kpreid alluded to.